There is two reason to pull out your weapon as a caster, granted they are poor reasons at best. You cannot block or parry with out your weapon out. As a mage, like I really care about this, but when soloing it does sometimes matter.
The 3MP per tick, I think it is better compared to your spells costs than max mana. 3 MP is 1.4% of Thunder (our cheapest spell) and 0.42% of Thundaga (our currently most expensive spell). Still 3MP per tick is very little, considering it takes 71 ticks to gt enough mana to cast thunder again. Still better than gaining no mana like the other classes though.